Hei.
If it is as CentoEvo says (4") I would recommend "Lightning Audio". Not a very known brand to me, but they handle very loud music without distortion. Crisp and clear midrange and highrange. 4" is also standard in my MkII Uno. Original was crap. I had two choises for 4", Sony X-plod and the Lightning Audio "Bolt" series. X-plod had crappy sound and could not handle loud music as well as the Bolt. Just a recommendation.
Morten.